<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8"/>
        <title>Document</title>
    </head>
    <body>
        <button id="btn1">按钮1</button>
        <button id="btn2">按钮2</button>
        <button onclick="demo()">按钮3</button>

        <script type="text/javascript">
            const btn1 = document.getElementById('btn1')
            btn1.addEventListener('click', () => {
                alert('按钮1被点击了')
            })

            const btn2 = document.getElementById('btn2')
            btn2.onclick = () => {
                alert('按钮2被点击了')
            }

            function demo() {
                alert('按钮3被点击了')
            }

        </script>
    </body>
</html>